The invention relates to a centrifugal pump having a double spiral housing including a rib configured as a dividing wall.
In centrifugal pumps there is known to be a hydraulic radial force produced by the interaction of the impeller and the pump housing. Such radial forces are subject to various influences, such as those described for example in KSB-Kreiselpumpenlexikon, 3rd Edition, 1989, pages 242 and 243. Single spiral housing pumps have in the design point along the impeller circumference a virtually constant pressure or velocity distribution. At this point a spiral housing pump can be operated virtually free of radial force. If, however, a spiral housing pump is operated at partial load or overload due to changed conditions of operation, this leads to increasing radial forces due to varying pressure or velocity distributions along the impeller circumference.
This disadvantage has led to the development of double spiral housings in which the radial forces are maintained at a low level over the entire operating range. Such double spiral housings, also known as twin volute pumps, are disclosed, for example by U.S. Pat. No. 2,955,540 to Pawlicki, and U.S. Pat. No. 3,289,598 to Buse et al., the disclosures of which are expressly incorporated by reference herein. A double spiral housing includes two spiral halves offset by 180.degree. in which a fluid flowing from the impeller is collected and fed to a common discharge connection. Due to the quasi mirror-image arrangement of the two spiral halves, an approximately symmetrical pressure distribution develops along one impeller circumference, whose resultant components of force cancel one another. A double spiral housing is created by inserting into a single spiral housing a so-called "rib" as a dividing wall which, as seen in the direction of impeller rotation forms a second half spiral beginning about 180.degree. from a lip of the housing. The side of the rib facing away from the impeller, the rear side, defines a diversion channel through which a fluid that has collected in the first spiral half is guided to the discharge connection.
The manufacturing of double spiral housings is technically complex and expensive, since the long housing cores necessary for casting are troublesome to secure so that they will not float up during the casting process. After the casting is complete the removal of the cores presents considerable difficulty in cleaning up the casting. For this purpose, additional holes are often provided in the pump housing wall, but it is difficult to weld those holes shut so as to withstand pressure. Despite these measures, the hard-to-reach areas of the double spiral housing allow only an incomplete surface treatment, the consequence of which is a loss of efficiency. To avoid these disadvantages, it is known from Pawlicki, U.S. Pat. No. 2,955,540, to make the rib as a separate part and insert it afterward into a cast spiral housing. This does facilitate machining the surface to dress the casting, but also requires a great amount of work in the production of a double spiral housing, so that there is no cost advantage.
For this reason much research has been conducted toward a critical study of the geometry of the ribs of double spiral housings and to examine their radial force curves. It is known, for example, from Buse et al., U.S. Pat. No. 3,289,598, to vary the length of the rib and the loop angle of the rib to obtain a change in the radial force curve. In FIG. 7 of U.S. Pat. No. 3,289,598, the radial force curves of various length ribs are shown. The shorter ribs achieve only an insignificant improvement in manufacture over the longer ribs; however, the shorter ribs disadvantageously increase the radial forces in the partial-load and overload ranges as compared with the longer ribs.